Conventionally, a thermal-type air flow measuring device for measuring an air flow rate by use of heat transfer between the device and air is widely known. The air flow measuring device is disposed in an intake passage leading to an internal combustion engine to be used for measuring the flow rate of intake air suctioned into the engine (flow rate of intake air may be hereinafter referred to as an intake air amount).
More specifically, this air flow measuring device takes in a part of intake air flowing through the intake passage and generates an electrical signal in accordance with the intake air amount. The air flow measuring device includes a housing that defines a bypass flow passage through which the taken-in intake air flows, and a sensor that is accommodated in the bypass flow passage to produce the electrical signal as a result of heat transfer between the sensor and the taken-in intake air. The air flow measuring device reduces the influence of turbulence of intake air in the intake passage by arranging the sensor in the bypass flow passage instead of disposing the sensor directly in the intake passage, through which intake air passes, so as to output a measurement value with few variations.
In addition, pulsation is inevitably caused in the intake air flowing through the intake passage in accordance with opening and closing of a valve of the engine. Accordingly, the intake air amount changes over time, fluctuating between a larger-side peak value of the pulsation and a smaller-side peak value of the pulsation. As a result, due to the thermal-type measuring method whereby the measured value is outputted using the heat transfer with air, the measured value is lower than a central value of the pulsation as a true value, and a negative-side error is thereby produced. Consequently, in the air flow measuring device, by making longer a passage length L2 of the bypass flow passage than a passage length L1 when air flows straight through the intake passage without flowing through the bypass flow passage, an increase range of the measurement value in accordance with a value of L2/L1 are set, and the negative-side error of the measurement value is thereby resolved.
The negative-side error varies according to the intake air amount, and the negative-side error becomes larger as the intake air amount is larger. For this reason, even if L2/L1 is set such that the negative-side error becomes 0 (Zero) with respect to when the intake air amount is a particular value, when the intake air amount changes from the particular value to the smaller-side, for example, the increase range due to L2/L1 becomes excessive. Therefore, conversely, the measured value including a positive-side error is outputted.
Thus, in the case of measurement of a flow rate of a flow with pulsation by the thermal-type air flow measuring device, due to the setting of L2/L1, upon variation of the flow rate on the smaller-side from a measuring range of the flow rate that can eliminate the negative-side error and positive-side error without excess or deficiency (hereinafter referred to as an error eliminable range), the positive-side error is produced in the measured value.
An air flow measuring device described in DE 102008042807 A1 includes a configuration for curbing generation of a vortical flow on a downstream side of an outlet of a bypass flow passage along an outer wall surface of a housing. More specifically, in the air flow measuring device in DE 102008042807 A1, two ribs parallel to the flow of intake air are provided so as to sandwich the outlet of the bypass flow passage between the ribs on the outer wall surface of the housing, and a cover that bridges between the two ribs on the downstream side of the outlet is further provided. Accordingly, it is described that the flow along the outer wall surface of the housing can be stabilized on the downstream side of the outlet as a result of the inhibition of the generation of the vortical flow. However, although the generation of the vortical flow can be limited, the configuration of DE 102008042807 A1 cannot deal with the generation of the positive-side error due to the variation of the intake air amount on the smaller-side.